This invention relates to a reciprocating internal-combustion engine of a low-temperature catalytic-combustion type and a combustion method therefor.
Generally speaking:
(A) Control of automotive exhaust emissions that constitute one source of atmospheric pollution is in most cases accomplished in the following ways:
(a) Use of a dilute gaseous mixture in an air-fuel ratio greater than the theoretical value (ca. 14.6 in weight ratio in case where the fuel is gasoline). Usually, the formation of the three pollutants (nitrogen oxides (NOx), unburned hydrocarbon (HC), and carbon monoxide (CO)) in the combustion process decreases as the mixture becomes dilute. Actually, however, a dilute mixture is difficult to ignite in such manner that the regulations on pollutions are cleared and often causes misfire. Efforts to overcome the difficulty have yielded a method of locally supplying an enriched mixture to the point where the electric spark is produced so as to obtain ready ignition and then allow the flame to spread toward the rest of the combustion space occupied by a more dilute mixture. The method makes possible the ignition and combustion of a gaseous mixture dilute as a whole, but it calls for special mechanism and complicated construction and operation, with a consequent increase in cost.
(b) Provision of a catalyst in the exhaust piping for decreases in the NOx, HC, and CO contents in exhaust emissions through oxidation or oxidation and reduction.
This increases the flow resistance of exhaust gases in the piping. In addition, the temperature of the catalyst frequently rises above 1000.degree. C., inviting dislocation in the crystal structure of the catalyst, particularly of the carrier with increased activity, causing rapid deterioration, and necessitating early catalyst replacement. For these and other reasons the engine performance is impaired and the operating conditions are restricted. Moreover, where the NOx are to be reduced with HC and CO, a lowered combustion efficiency will result. Thus, the use of a catalyst is costly in every point.
(B) One of the factors most responsible for the pollution of exhaust gases from internal-combustion engines is the incomplete combustion of the air-fuel mixture which is caused in the flame-absent or quench layer.
The term "quench layer" herein used indicates a very thin layer of the gaseous mixture formed adjacent to the combustion chamber surfaces by the cooling action of the engine, i.e., of the surrounding metal walls, and which is discharged out of the chamber before the flame propagates to that layer. This unburned gaseous mixture is partly burned as it is exposed to hot gases on its way to the atmosphere, but most is incompletely burned and diffused into the exhaust gases, rendering its recombustion or recovery difficult.
In the combustion chamber, with the piston at its top dead center (TDC), the quench layer is mainly in contact with the surrounding surfaces of the combustion chamber, formed by the inner walls of the cylinder head, piston head, and cylinder wall, which are separate from the piston-sliding region, and the layer cannot burn. Thus, under normal operating conditions, or while the cylinder is being regularly cooled, this layer is believed to be at temperatures too low for ignition (that is, below 250.degree. C.; particularly, in the range of 200.degree. C. to 250.degree. C. and especially in the initial stages of combustion). Reconsideration of these facts has led to the present invention.
If the wall surfaces mentioned above are utilized as areas on which catalytic combustion takes place, then it will be possible to control the reaction starting temperature and the catalyst temperature within desirable ranges by choosing proper thickness and shape for the catalyst layers. It is no longer essential to find out and employ a catalyst that will stand elevated temperatures (over 1000.degree. C.), and the objects of the invention to be described in detail below can be realized easily and economically, without the need of substantially modifying the construction of existing conventional engines.